Microelectromechanical systems (MEMS), for example, gyroscopes, resonators and accelerometers, utilize micromachining techniques (i.e., lithographic and other precision fabrication techniques) to reduce mechanical components to a scale that is generally comparable to microelectronics. MEMS typically include a mechanical structure fabricated from or on, for example, a silicon substrate using micromachining techniques.
With surface micromachining, a MEMS device structure can be built on a silicon substrate using processes such as chemical vapor deposition. These processes allow MEMS structures to include layer thicknesses of less than a few microns with substantially larger in-plane dimensions. Frequently, these devices include parts which are configured to move with respect to other parts of the device. In this type of device, the movable structure is frequently built upon a sacrificial layer of material. After the movable structure is formed, the movable structure can be released by selective wet etching of the sacrificial layers in aqueous hydrofluoric acid (HF). After etching, the released MEMS device structure can be rinsed in deionized water to remove the etchant and etch products.
Due to the large surface area-to-volume ratio of many movable structures, a MEMS device including such a structure is susceptible to interlayer or layer-to-substrate adhesion during the release process (release adhesion) or subsequent device use (in-use adhesion). This adhesion phenomenon is more generally called stiction. Stiction is exacerbated by the ready formation of a 5-30 angstrom thick native oxide layer on the silicon surface, either during post-release processing of the MEMS device or during subsequent exposure to air during use. Silicon oxide is hydrophilic, encouraging the formation of water layers on the native oxide surfaces that can exhibit strong capillary forces when the small interlayer gaps are exposed to a high humidity environment. Furthermore, Van der Waals forces, due to the presence of certain organic residues, hydrogen bonding, and electrostatic forces, also contribute to the interlayer attraction. These cohesive forces can be strong enough to pull the free-standing released layers into contact with another structure, causing irreversible latching and rendering the MEMS device inoperative.
Various approaches have been used in attempts to minimize adhesion in MEMS devices. These approaches include drying techniques, such as freeze-sublimation and supercritical carbon dioxide drying, which are intended to prevent liquid formation during the release process, thereby preventing capillary collapse and release adhesion. Vapor phase HF etching is commonly used to alleviate in-process stiction. Other approaches are directed to reducing stiction by minimizing contact surface areas, designing MEMS device structures that are stiff in the out-of-plane direction, and hermetic packaging.
An approach to reducing in-use stiction and adhesion issues is based upon surface modification of the device by addition of an anti-stiction coating. The modified surface ideally exhibits low surface energy by adding a coating of material, thereby inhibiting in-use adhesion in released MEMS devices. Most coating processes have the goal of producing a thin surface layer bound to the native silicon oxide that presents a hydrophobic surface to the environment. In particular, coating the MEMS device surface with self-assembled monolayers (SAMs) having a hydrophobic tail group has been shown to be effective in reducing in-use adhesion. SAMs have typically involved the deposition of organosilane coupling agents, such as octadecyltrichlorosilane and perfluorodecyltrichlorosilane, from nonaqueous solutions after the MEMS device is released. Even without anti-stiction coating, native oxide generation occurs on silicon surfaces.
In spite of these various approaches, in-use adhesion remains a serious reliability problem with MEMS devices. One aspect of the problem is that even when an antistiction coating is applied, the underlying silicon layer may retain various charges. For example, silicon by itself is not a conductor. In order to modify a silicon structure to be conductive, a substance is doped into the silicon. The realizable doping-level is limited, however, due to induced stress in the functional silicon layer. Accordingly, during manufacturing process, charges are deposited on the silicon surfaces of sensing elements and the charges do not immediately migrate. The charges include dangling bonds due to trench forming processes used to define various structures. In capacitive sensing devices those charges may cause a reliability issue since they are not all locally bound. Some charges have a certain mobility and may drift as a function of temperature or aging. This can lead to undesired drift effects, e. g. of the sensitivity or offset of the capacitive sensor. Therefore, a highly conductive working layer (not possible w/silicon) or at least a highly conductive coating on top of the structures in order to not accumulate surface charges would be desirable.
Moreover, the limited conductivity of silicon may result in unacceptable RC time constants in electronic evaluation circuits including capacitive sensors. A sensor element with, e. g., a 10 pF total capacitance (C) and 10 kOhm total resistance (R) may be limited to operation below frequencies of about 1 MHz. Operation at higher frequencies is desired in certain applications, however, since higher frequency operation may lead to a better signal to noise performance of the sensor. Therefore, increased conductivity in MEMS devices which enable achievement of lower RC time constants would be beneficial.
Thus, there remains a need for a reliable structure for MEMS devices that is compatible with MEMS fabrication processes that can be used to reduce stiction forces, surface charges, and/or the resistivity of MEMS structures.